Dynamically generate bmc hostnames from static inventory? - ansible

Question

I have an ansible inventory that looks something like:
all:
children:
nodes:
vars:
bmc_user: root
bmc_pass: secret
hosts:
node1.example.com:
bmc_addr: node1-bmc.example.com
node2.example.com:
bmc_addr: node2-bmc.example.com
For running something like ipmitool, I can simply delegate the task to localhost, like this:
- hosts: all
gather_facts: false
tasks:
- delegate_to: localhost
command: >-
ipmitool -I lanplus -H {{ bmc_addr }} -U {{ bmc_user }} -P {{ bmc_pass }} power off
Occasionally, I need to interact with the bmc using ssh. Thsi is
tricky, because typically this requires password authentication. I
want to write something like this:
- hosts: all
gather_facts: false
tasks:
- delegate_to: "{{ bmc_addr }}"
command: >-
racadm set iDRAC.IPMILan.Enabled Enable
...but this won't work, because the credentials required to access the
bmc are not the same as the credentials necessary to access the host.
I would like to be able to do something like this in my inventory:
all:
children:
nodes:
vars:
bmc_user: root
bmc_pass: secret
hosts:
node1.example.com:
bmc_addr: node1-bmc.example.com
node2.example.com:
bmc_addr: node2-bmc.example.com
nodes_bmc:
vars:
ansible_user: root
ansible_ssh_pass: secret
hosts:
node1-bmc.example.com:
node2-bmc.example.com:
...but I would like the hosts in the nodes_bmc group to be populated
dynamically from the bmc_addr values in the nodes group, rather
than having to specify them manually.
I've been puzzling over this for a bit, and the only solution I've
come up with is to write some tooling that reads the ansible inventory
and then generates a new static inventory containing the necessary
entries. That seems clunky, and requires ensuring that the "generate
bmc inventory" tool is run whenever the main inventory is modified.

Answer 1

Let's try the add_host module:
- name: add hosts
hosts: all
gather_facts: no
tasks:
- add_host:
hostname: "{{ hostvars[item].bmc_addr }}"
groups:
- nodes_bmc
loop: "{{ play_hosts }}"
Now, you need a new play so you have all the hosts in the play.
- name: Show all hosts
hosts: all
gather_facts: no
tasks:
- debug:
var: play_hosts
run_once: yes
Running with your inventory (but w/o any hosts in the node_bmc group):
$ ansible-playbook -i hosts.yml ./add_hosts.yml
PLAY [add hosts] ***********************************************************************************************************
TASK [add_host] ************************************************************************************************************
ok: [node1.example.com] => (item=node1.example.com)
ok: [node1.example.com] => (item=node2.example.com)
PLAY [Show all hosts] ******************************************************************************************************
TASK [debug] ***************************************************************************************************************
ok: [node1-bmc.example.com] => {
"play_hosts": [
"node1-bmc.example.com",
"node2-bmc.example.com",
"node1.example.com",
"node2.example.com"
]
}
PLAY RECAP *****************************************************************************************************************
node1-bmc.example.com : ok=1 changed=0 unreachable=0 failed=0 skipped=0 rescued=0 ignored=0
node1.example.com : ok=1 changed=0 unreachable=0 failed=0 skipped=0 rescued=0 ignored=0

Answer 2

Now, here's a totally different way.
You will have two file in your inventory:
The one you currently have (but w/o the hosts: section under nodes_bmc)
A generate_bmc.sh file, which must be executable: chmod +x generate_bmc.sh
That file will simply be this:
#!/bin/bash
# Figure out where we are
here=$( dirname $0 )
# Write header
cat << 'end_header'
{
"nodes_bmc": {
"hosts": [
end_header
# Get hosts from inventory file
hosts=$( ansible-inventory --list -i ${here}/hosts.yml | grep '"bmc_addr":' | cut -d: -f2 | sed 's/,$//' )
# Or, if you have jq installed:
# hosts=$( ansible-inventory --list -i ${here}/hosts.yml | jq "._meta.hostvars[].bmc_addr" )
last=$( echo $hosts | sed 's/.* //' )
# Write hosts
for host in ${hosts}
do
echo -n " "$host
[ "${host}" != "${last}" ] && echo ","
done
echo
# Close output
cat << 'end_footer'
]
}
}
end_footer
In your ansible.cfg file, have the line:
inventory = /the/directory/that/has/those/two/files
You can test this with ansible-inventory --list.